Doppler methods in medical ultrasound encompass a number of related techniques for imaging and quantifying blood flow. For stationary targets, the round trip travel time of an ultrasound pulse transmitted from a transducer, reflected from the target, and returned back to the transducer is the same for each transmitted pulse. In the case of a moving object, successive echographic returns will arrive at different times with respect to the transmit pulse. For example, echographic returns that are received at intervals less than the stationary round-trip time may represent reflectors moving towards the TX/RX probe, while returns received at intervals longer than the stationary round-trip time may represent reflectors moving away from the TX/RX probe. This is the result of the well-known Doppler Effect, which may also be described in terms of relative frequencies. In the frequency domain reflected signals received at a higher-than-expected frequency may represent reflectors moving towards the transmitter/receiver, while reflected signals received at a lower-than-expected frequency may represent reflectors moving away from the transmitter/receiver. From this information, the velocity of the moving reflector can be estimated.
Conventional ultrasound (or “scanline based” ultrasound as used herein) utilizes a phased array controller to produce and steer a substantially linear transmit waveform. In order to produce a B-mode image, a sequence of such linear waveforms (or “scanlines”) may be produced and steered so as to scan across a region of interest. Echoes are received along each respective scanline. The individual scanlines may then be combined to form a complete image.
Because a traditional scanline-based ultrasound path is directional (along the scanline axis), only motion along a scanline axis produces a Doppler (motion) signal. Flow that is transverse to the scanline is not detectable using such conventional methods, and thus the velocity magnitudes obtained in conventional Doppler methods represent only the component of the flow velocity vector that lies along the transmit/receive scanline axis. In order to estimate the true magnitude of the flow velocity vector, Vector Doppler methods are employed. These methods rely on data from multiple intersecting scanlines to estimate the direction of the flow vector and the flow velocity vector.
Several scanline-based Doppler methods have been developed to present different aspects of blood flow. Typically, “spatial imaging” (otherwise referred to as “B-mode” imaging or “sector scan” imaging) of the flow field is used to locate vessels, to measure their size, and to observe flow structure. “Flow imaging” is used in conjunction with echographic imaging in a “duplex” mode that combines both types of images in an overlay, with echographic amplitude presented in grayscale and flow velocity rendered in color.
A sonographer may obtain a detailed quantification of flow velocity by selecting a much smaller sample volume chosen within the region of interest. The smallest volume that can be sampled and processed independently is given by the axial length (the transmit pulse length) and the lateral beam widths (in and out of the imaging plane) of the scanline beam. Using scanline-based Doppler methods, this small sample volume, also known as a “range gate,” a “Doppler gate” or a “Doppler range gate” must be defined by a sonographer via a user interface prior to transmission and receipt of Doppler ultrasound signals. This requirement for pre-defining a Doppler range gate means that moving reflectors that lie outside of the pre-defined range gate may not be identified without defining a new range gate, which may require conducting a separate Doppler imaging session.
Scanline-based Doppler imaging can also impose substantial limits on the frame-rate of B-mode images within a scanline-based ultrasound imaging system. The frame rate of a scanline-based ultrasound imaging system is the pulse-repetition frequency (PRF, which is limited by the round-trip travel time of ultrasound in the imaged medium) divided by the number of scanlines per frame. Typical scanline-based ultrasound imaging systems use between about 64 and about 192 scanlines per frame. Typically, an ensemble of between 8 and 32 pulse-echo events is used for each Doppler scanline in the region of interest. Such Doppler ensembles are effectively an interruption to a B-mode sector scan, resulting in a lower B-mode frame rate (or requiring fewer scanlines per B-mode frame) than the system would otherwise be capable of.